This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-242865, filed Aug. 10, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a waveform measuring apparatus. More particularly, the present invention relates to a waveform measuring apparatus for measuring a signal waveform of a signal under test having an input arbitrary repetition cycle.
2. Description of Related Art
Conventionally, there have been proposed a variety of measuring techniques for measuring a signal waveform of a signal under test such as an electrical signal or an optical signal and the like having an input arbitrary repetition cycle.
However, in the case of a high frequency synthesized signal of which a repetition cycle of a signal under test, i.e., a repetition frequency exceeds 10 GHz, the signal waveform of the signal under test cannot be directly observed on a display screen of an oscilloscope or the like. Thus, the selection range of such waveform measuring technique is limited itself.
A conventional typical technique for measuring a signal waveform of a signal under test of which the repetition frequency exceeds 10 GHz will be described with reference to FIGS. 7A, 7B, and 7C.
As shown in FIGS. 7A and 7B, a signal under test xe2x80x9caxe2x80x9d having a repetition cycle Ta (for example, repetition frequency fa=10 GHz) is sampled by means of a sampling signal xe2x80x9cbxe2x80x9d having a cycle Tb (for example, repetition frequency xe2x80x9cfbxe2x80x9d=999.9 MHz) longer than the repetition cycle Ta of this signal under test xe2x80x9caxe2x80x9d.
In this case, a mutual relationship between the repetition cycle Ta of the signal under test xe2x80x9caxe2x80x9d and the cycle Tb that is longer than the repetition cycle Ta of the signal under test xe2x80x9caxe2x80x9d is adjusted. As shown in FIGS. 7A and 7B, with an elapse of time, a sampling position of the sampling signal xe2x80x9cbxe2x80x9d in the signal waveform in the repetition cycle Ta of the signal under test xe2x80x9caxe2x80x9d is shifted by a very small amount of time xcex94T, so that the time is delayed by 2xcex94T, 3xcex94T, 4xcex94T, 5xcex94T, 6xcex94T, . . . nxcex94T.
Therefore, a signal under test xe2x80x9ccxe2x80x9d after sampled by this sampling signal xe2x80x9cbxe2x80x9d is obtained as a discrete waveform of which a pulse shaped waveform is generated at a position synchronized with the sampling signal xe2x80x9cbxe2x80x9d, as shown in FIG. 7C.
Then, the enveloped waveform of such each pulse waveform is obtained as a signal waveform xe2x80x9cdxe2x80x9d extended in a time axis direction of the signal under test xe2x80x9caxe2x80x9d.
Based on the sampling technique shown in FIGS. 7A, 7B, and 7C, a waveform measuring apparatus for measuring a signal waveform xe2x80x9cdxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d is configured as shown in FIG. 8.
The signal under test xe2x80x9caxe2x80x9d having the repetition cycle Ta (repetition frequency xe2x80x9cfaxe2x80x9d) is input to a sampling circuit 1 and a timer circuit 2.
The timer circuit 2 outputs a timing of generating each pulse of the sampling signal xe2x80x9cbxe2x80x9d to a sampling signal circuit 3.
In this case, the timing of generating each pulse of the sampling signal xe2x80x9cbxe2x80x9d is required to be shifted by xcex94T from a start time of the repetition cycle Ta of the signal under test xe2x80x9caxe2x80x9d, and thus, is synchronized with the signal under test xe2x80x9caxe2x80x9d having an input reference time signal.
The sampling signal generator circuit 3 applies to the sampling circuit 1 a sampling signal xe2x80x9cbxe2x80x9d having a pulse form every time a pulse generation timing is input from the timer circuit 2.
The sampling circuit 1 samples the input signal under test xe2x80x9caxe2x80x9d by means of the sampling signal xe2x80x9cbxe2x80x9d input from the sampling signal generator circuit 3, and delivers the sampled signal under test xe2x80x9ccxe2x80x9d to a signal processing/waveform display section 4.
The signal processing/waveform display section 4 calculates an enveloped waveform of the sampled signal under test xe2x80x9ccxe2x80x9d, converts a scale of the time axis of this enveloped waveform to a scale of the original signal under test xe2x80x9caxe2x80x9d, and displays and outputs a signal waveform xe2x80x9cdxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d.
FIG. 9 is a block diagram depicting a schematic configuration of another conventional waveform measuring apparatus for measuring a signal waveform xe2x80x9cdxe2x80x9d of a signal under test xe2x80x9caxe2x80x9d by using a principle of the sampling technique shown in FIGS. 7A, 7B, and 7C.
The signal under test xe2x80x9caxe2x80x9d having the repetition frequency xe2x80x9cfaxe2x80x9d (repetition cycle Ta) is input to the sampling circuit 1 and a frequency divider 5.
The frequency divider 5 divides the repetition frequency xe2x80x9cfaxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d by 1/n, and delivers it to a phase comparator 6.
A voltage control oscillator (VCO) 7 generates a signal having a frequency (fa/n) that is 1/n (n: positive integer) of the repetition frequency xe2x80x9cfaxe2x80x9d, and feeds it back to the phase comparator 6.
The phase comparator 6 detects a phase difference between a phase of an output signal of the voltage control oscillator and a phase of an output signal of the frequency divider 5, and delivers the phase difference signal to the voltage control oscillator (VCO).
By means of such a phase synchronization loop (PLL), the phase of the output signal from the voltage control oscillator (VCO) 7 is controlled so as to be synchronized with the phase of the signal under test xe2x80x9caxe2x80x9d.
An output signal having a frequency (fa/n) output from the voltage control oscillator (VCO) 7 is converted into a frequency of (fa/n)xe2x88x92xcex94f by means of a next frequency divider 8a having a fixed rate of frequency dividing and a multiplier 8b having a fixed rate of frequency multiplying, and the converted signal is input to a sampling signal generator circuit 3a. 
The sampling signal generator circuit 3a applies to the sampling circuit 1 the sampling signal xe2x80x9cbxe2x80x9d having a repetition frequency synchronized with an
input signal fb=fa/n)xe2x88x92xcex94fxe2x80x83xe2x80x83(1)
and a repetition
cycle of Tb=(nTa)+xcex94Txe2x80x83xe2x80x83(2).
However, a relationship between xcex94f and xcex94T is approximately shown by the formula below.
xcex94f/xcex94T=fa2/n2xe2x80x83xe2x80x83(3)
The sampling circuit 1 samples the input signal under test xe2x80x9caxe2x80x9d by means of the sampling signal xe2x80x9cbxe2x80x9d input from the sampling signal generator circuit 3a, and delivers the sampled signal under test xe2x80x9ccxe2x80x9d to the signal processing/waveform display section 4.
The signal processing/waveform display section 4 calculates an enveloped waveform of the input, sampled signal under test xe2x80x9ccxe2x80x9d, converts a scale of a time axis of this enveloped waveform into a scale of the original signal under test xe2x80x9caxe2x80x9d, and displays and outputs a signal waveform xe2x80x9cdxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d.
In this case, a magnification rate of the signal under test xe2x80x9caxe2x80x9d of the measured, enveloped waveform to the signal waveform xe2x80x9cdxe2x80x9d is defined as (fa/nxcex94f)
In the case where the signal under test xe2x80x9caxe2x80x9d is obtained as an optical signal instead of an electrical signal, this optical signal is converted into an electrical signal, and the converted signal is applied to the timer circuit 2 or frequency divider 5.
In addition, in the case of the optical signal, an electro-absorption modulator is employed instead of the sampling circuit 1.
This electro-absorption modulator applies a pulse shaped electric field caused by a sampling signal to a forwarding direction of an incident optical signal, thereby making it possible to sample a pulse shaped signal under test xe2x80x9caxe2x80x9d that consists of an input optical signal.
Then, the signal under test xe2x80x9ccxe2x80x9d such as the sampled optical signal is converted into an electrical signal, and the converted signal is delivered to the signal processing/waveform display section 4.
However, in a conventional measuring apparatus employing a sampling technique shown in FIGS. 8 and 9 as well, there has been the following problems to be solved.
That is, in the waveform measuring apparatus shown in FIG. 8, an output timing of a timing signal output by the timer circuit 2 is set by employing a reference time signal output from an oscillator provided in this timer circuit 2.
In this case, a reference time signal output from the oscillator and a reference signal for determining a repetition frequency Ta of the signal under test xe2x80x9caxe2x80x9d are signals generated by means of different signal generators in view of hardware. Thus, the verified phases are not always coincident with each other at all the timings.
Thus, there is a problem that a large jitter occurs with the enveloped waveform of the signal under test xe2x80x9ccxe2x80x9d after sampled by means of the sampling circuit 1, and then, the measurement precision of the signal waveform xe2x80x9cdxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d is lowered.
For example, in the case where the repetition frequency Fa of the signal under test xe2x80x9caxe2x80x9d is about 10 GHz, a quantity of generated jitter reaches several ps (pico-seconds). In addition, in the signal waveform xe2x80x9cdxe2x80x9d of the final signal under test xe2x80x9caxe2x80x9d considering a bandwidth restriction of the photo detector and electrical processing circuit, the waveform tolerance reaches 10 ps to 20 ps.
Further, in the waveform measuring apparatus shown in FIG. 9, an output signal from the multiplier 8b for generating the sampling signal xe2x80x9cbxe2x80x9d having the repetition frequency fb=(fa/n)xe2x88x92xcex94f output from the sampling signal generator circuit 3a is generated by means of a PLL circuit composed of a frequency divider 5 for frequency dividing the signal under test xe2x80x9caxe2x80x9d, a phase comparator 6, and a voltage control oscillator (VCO) 7.
That is, in this case, the sampling signal xe2x80x9cbxe2x80x9d is generated by processing the signal under test xe2x80x9caxe2x80x9d targeted to be measured. Thus, the sampling signal xe2x80x9cbxe2x80x9d is always phase synchronized with the signal under test xe2x80x9caxe2x80x9d.
Therefore, in the waveform measuring apparatus shown in FIG. 9, unlike the waveform measuring apparatus shown in FIG. 8, there is no need to additionally provide an oscillator for generating the sampling signal xe2x80x9cbxe2x80x9d. Thus, a quantity of generated jitter is remarkably restrained.
However, the repetition frequency xe2x80x9cfbxe2x80x9d of the sampling frequency xe2x80x9cbxe2x80x9d is designates by a function of the repetition frequency xe2x80x9cfaxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d, as is evident from the previously described formulas (1) and (3).
Thus, in the waveform measuring apparatus having a fixed rate of frequency dividing and a fixed rate of frequency multiplying, the repetition frequency xe2x80x9cfbxe2x80x9d of the sampling signal xe2x80x9cbxe2x80x9d cannot be arbitrarily set independently of the repetition frequency xe2x80x9cfaxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d.
Namely, if the repetition frequency xe2x80x9cfaxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d changes, the time resolution of the signal waveform xe2x80x9cdxe2x80x9d of the measured, signal under test xe2x80x9caxe2x80x9d, i.e., measurement precision automatically changes.
This fact designates that the signal waveform xe2x80x9cdxe2x80x9d of the signal under test xe2x80x9caxe2x80x9d cannot be measured by an arbitrary time resolution.
In addition, there is a problem that the selection range of the repetition frequency xe2x80x9cfbxe2x80x9d of the sampling signal xe2x80x9cbxe2x80x9d is significantly restricted due to the specification or characteristics of each of the frequency dividers 5 and 8a . 
The present invention has been made in order to solve the foregoing problem. It is an object of the present invention to provide a waveform measuring apparatus for generating a frequency synthesized signal for acquiring a frequency of a sampling signal by employing a frequency synthesized signal generator, and always phase synchronizing the thus generated frequency synthesized signal with a signal under test by employing a PLL technique, whereby a frequency of a sampling signal for sampling the signal under test can be arbitrarily set independent of a repetition frequency of the signal under test, the measurement precision of the signal waveform of the signal under test can be improved while a quantity of generated jitter is remarkably restrained, and the signal waveform can be measured with an arbitrary resolution precision.
The present invention is applicable to a waveform measuring apparatus for sampling a signal under test having an input arbitrary repetition cycle by means of a sampling signal having a cycle longer than the repetition cycle of this signal under test, obtaining an enveloped waveform of the thus sampled signal under test, and obtaining a signal waveform of the signal under test from the enveloped waveform.
According to a first aspect of the present invention, there is provided a waveform measuring apparatus having a sampling section (12) for sampling a signal under test having an input arbitrary repetition cycle by means of a sampling signal having a cycle longer than the repetition cycle of the signal under test and a data processing section (23) for obtaining an enveloped waveform of a signal under test sampled by means of the sampling section, and obtaining a signal waveform of the signal under test from the enveloped waveform, the waveform measuring apparatus comprising:
a frequency synthesized signal generator (15) having a reference signal input terminal (REF), the frequency synthesized signal generator outputting a frequency synthesized signal having a frequency equal to a repetition frequency of the signal under test by employing a reference signal applied to the reference signal input terminal;
a phase comparator (14) detecting a phase difference between a phase of the frequency synthesized signal output from the frequency synthesized signal generator and a phase of the signal under test, thereby outputting a phase difference signal;
a voltage control oscillator (17) for generating a reference signal phase synchronized with the signal under test based on the phase difference signal output from the phase comparator, and feeding the reference signal back to the reference signal input terminal of the frequency synthesized signal generator and the reference signal input terminal of the phase comparator; and
a sampling signal generator circuit (18) for generating the sampling signal by employing a reference signal output from the voltage control oscillator.
In the thus configured waveform measuring apparatus, a sampling signal generated by a sampling signal generating circuit is generated based on a reference signal output from a voltage control oscillator.
Thus, the waveform measuring apparatus can be set to an arbitrary frequency independent of a repetition frequency of a signal under test by means of a frequency synthesized signal generator provided in the sampling signal generating circuit.
Furthermore, a phase of this reference signal is always phase synchronized with a phase of a signal under test by means of a PLL circuit composed of a frequency synthesized signal generator, a phase comparator, and a voltage control oscillator.
Therefore, a frequency of a sampling signal for sampling a signal under test can be arbitrarily set independent of a repetition frequency of a signal under test, and is always phase synchronized with the signal under test. Thus, an occurrence of a jitter is restrained.
In addition, in order to achieve the foregoing object, according to a second aspect of the present invention, there is provided a waveform measuring apparatus according to the first aspect, further comprising:
a frequency measuring section (26) for measuring a repetition frequency of the signal under test based on the reference signal output from the voltage control oscillator; and
a frequency setting section (16) for setting a frequency of a frequency synthesized signal output from the frequency synthesized signal generator to the frequency synthesized signal generator based on a repetition frequency of the signal under test measured at the frequency measuring section.
In the thus configured waveform measuring apparatus, even if the repetition of a signal under test is unknown, this repetition frequency is measured by the frequency measuring section, and an output frequency synthesized signal of a frequency synthesized signal generator is automatically set based on the thus measured repetition frequency.
Therefore, even if the repetition frequency of the signal under test is unknown, the signal waveform of the signal under test can be precisely measured.
In addition, in order to achieve the foregoing object, according to a third aspect of the present invention, there is provided a waveform measuring apparatus according to the first aspect, further comprising:
a power divider (11) for power dividing the signal under test that is an optical signal in two ways, if the signal under test is the optical signal; and
a clock recovery (13) for detecting a clock with its repetition cycle from one of the signal under test divided by the power divider, thereby outputting the signal under test that is the optical signal to be converted into a signal under test of an electrical signal of a sine waveform having the repetition frequency.
In addition, in order to achieve the foregoing object, according to a fourth aspect of the present invention, there is provided a waveform measuring apparatus according to the third aspect, further comprising:
a photo detector (21) for, when the sampling section is an electro-absorption modulator (12), light receiving a signal under test obtained by sampling a signal under test of the optical signal input from the power divider by means of a sampling signal input from the sampling signal generator circuit, and converting a signal under test that is an optical signal after sampled into a signal under test of an electrical signal;
an analog/digital converter (22) for converting the signal under test converted into the electrical signal by means of the photo detector into a digital signal under test, and delivering the converted digital signal under test to the data processing section; and
a display (24) for converting a scale of a time axis of the enveloped waveform obtained by the data processing section into a scale of an original signal under test, and displaying a signal waveform of the signal under test.
In addition, in order to achieve the foregoing object, according to a fifth aspect of the present invention, there is provided a waveform measuring apparatus having a sampling section (12) for sampling a signal under test having an input arbitrary repetition cycle by means of a sampling signal having a cycle longer than the repetition cycle of the signal under test and a data processing section (23) for obtaining an enveloped waveform of a signal under test sampled by means of the sampling section, and obtaining a signal waveform of the signal under test from the enveloped waveform, the waveform measuring apparatus comprising:
a frequency synthesized signal generator (15) having a reference signal input terminal (REF), the frequency synthesized signal generator outputting a frequency synthesized signal having a frequency obtained by adding or subtracting a frequency of a reference signal to a repetition frequency of the signal under test by employing the reference signal applied to the reference signal input terminal;
a mixer (27) for mixing a frequency synthesized signal output from the frequency synthesized signal generator with the signal under test;
a phase comparator (14) having a reference signal input terminal (REF), the comparator detecting a phase difference between a phase of a reference signal applied to the reference signal input terminal and a phase of an output signal from the mixer, thereby outputting a phase difference signal;
a voltage control oscillator (17) for generating a reference signal phase synchronized with the signal under test based on the phase difference signal output from the phase comparator, and feeding the reference signal back to the reference signal input terminal of the frequency synthesized signal generator and the reference signal input terminal of the phase comparator; and
a sampling signal generator circuit (18) for generating the sampling signal by employing a reference signal output from the voltage control oscillator.
In the thus configured waveform measuring apparatus, a sampling signal generated by sampling signal generating means is generated based on a reference signal output from a voltage control oscillator.
In the thus configured waveform measuring apparatus, a mixer is interposed between a frequency synthesized signal generator and a phase comparator.
The mixer signal combines a frequency synthesized signal and a signal under test output from a frequency synthesized signal generator, and delivers these signals to a phase comparator.
A frequency of a frequency synthesized signal generated by a frequency synthesized signal generator is set to a frequency obtained by adding or subtracting a repetition frequency of a signal under test and a frequency of a reference signal with each other, whereby a frequency component of an output signal delivered from the mixer to a phase comparator can be lowered to a frequency component of a reference signal. Thus, there is no need to employ a phase comparator capable of fast processing.
In addition, in order to achieve the foregoing object, according to a sixth aspect of the present invention, there is provided a waveform measuring apparatus according to the fifth aspect, further comprising:
a frequency measuring section for measuring a repetition frequency of the signal under test; and
a frequency setting section for setting a frequency of a frequency synthesized signal output from the frequency synthesized signal generator to the frequency synthesized signal generator based on a repetition frequency of the signal under test measured at the frequency measuring section.
In addition, in order to achieve the foregoing object, according to a seventh aspect of the present invention, there is provided a waveform measuring apparatus according to the fifth aspect, further comprising:
a power divider for power dividing the signal under test that is an optical signal in two ways, if the signal under test is the optical signal; and
a clock recovery for detecting a clock with its repetition cycle from one of the signal under test divided by the power divider, thereby outputting the signal under test that is the optical signal to be converted into a signal under test of an electrical signal of a sine waveform having the repetition frequency.
In addition, in order to achieve the foregoing object, according to an eighth aspect of the present invention, there is provided a waveform measuring apparatus according to the seventh aspect, further comprising:
a photo detector for, when the sampling section is an electro-absorption modulator, light receiving a signal under test obtained by sampling a signal under test of the optical signal input from the power divider by means of a sampling signal input from the sampling signal generator circuit, and converting a signal under test that is an optical signal after sampled into a signal under test of an electrical signal;
an analog/digital converter for converting the signal under test converted into the electrical signal by means of the photo detector into a digital signal under test, and delivering the converted digital signal under test to the data processing section; and
a display for converting a scale of a time axis of the enveloped waveform obtained by the data processing section into a scale of an original signal under test, and displaying a signal waveform of the signal under test.
In addition, in order to achieve the foregoing object, according to a ninth aspect of the present invention, there is provided a waveform measuring apparatus having a sampling section for sampling a signal under test having an input arbitrary repetition cycle by means of a sampling signal having a cycle longer than the repetition cycle of the signal under test and a data processing section for obtaining an enveloped waveform of a signal under test sampled by means of the sampling section, and obtaining a signal waveform of the signal under test from the enveloped waveform, the waveform measuring apparatus comprising:
a frequency synthesized signal generator having a reference signal input terminal, the frequency synthesized signal generator outputting a frequency synthesized signal having a frequency equal to 1/n (n: positive integer) of a repetition frequency of the signal under test by employing a reference signal applied to the reference signal input terminal;
a frequency divider (28) for frequency dividing a repetition frequency of the signal under test to 1/n (n: positive integer), and outputting the signal under test;
a phase comparator for detecting a phase difference between a phase of the frequency synthesized signal output from the frequency synthesized signal generator and a phase of the signal under test frequency-divided to 1/n (n: positive integer), and outputting a phase difference signal;
a voltage control oscillator for generating a reference signal phase-synchronized with the signal under test based on the phase difference signal output from the phase comparator, and feeding the reference signal back to the reference signal input terminal of the frequency synthesized signal generator; and
a sampling signal generator circuit for generating the sampling signal by employing the reference signal output from the voltage control oscillator.
In addition, in order to achieve the foregoing object, according to a tenth aspect of the present invention, there is provided a waveform measuring apparatus according to the ninth aspect, further comprising:
a power divider for power dividing the signal under test that is an optical signal in two ways, if the signal under test is the optical signal; and
a clock recovery for detecting a clock with its repetition cycle from one of the signal under test divided by the power divider, thereby outputting the signal under test that is the optical signal to be converted into a signal under test of an electrical signal of a sine waveform having the repetition frequency.
In addition, in order to achieve the foregoing object, according to an eleventh aspect of the present invention, there is provided a waveform measuring apparatus according to the tenth aspect, further comprising:
a photo detector for, when the sampling section is an electro-absorption modulator, light receiving a signal under test obtained by sampling a signal under test of the optical signal input from the wave divider by means of a sampling signal input from the sampling signal generator circuit, and converting a signal under test that is an optical signal after sampled into a signal under test of an electrical signal;
an analog/digital converter for converting the signal under test converted into the electrical signal by means of the photo detector into a digital signal under test, and delivering the converted digital signal under test to the data processing section; and
a display for converting a scale of a time axis of the enveloped waveform obtained by the data processing section into a scale of an original signal under test, and displaying a signal waveform of the signal under test.
In addition, in order to achieve the foregoing object, according to a twelfth aspect of the present invention, there is provided a waveform measuring apparatus having a sampling section for sampling a signal under test having an input arbitrary repetition cycle by means of a sampling signal having a cycle longer than the repetition cycle of the signal under test and a data processing section for obtaining an enveloped waveform of a signal under test sampled by means of the sampling section, and obtaining a signal waveform of the signal under test from the enveloped waveform, the waveform measuring apparatus comprising:
a frequency synthesized signal generator having a reference signal input terminal, the frequency synthesized signal generator outputting a frequency synthesized signal having a frequency obtained by adding or subtracting a frequency equal to a repetition frequency of the signal under test and a frequency that is m times (m: positive integer) of the frequency of the reference signal;
a frequency multiplier (29) for outputting a signal having a frequency that is m times (m: positive integer) of the frequency of the reference signal;
a mixer for mixing the frequency synthesized signal output from the frequency synthesized signal generator with the signal under test;
a phase comparator having a reference signal input terminal, the phase comparator generating a phase difference between a phase of a signal having a frequency that is m times of the frequency of the reference signal applied from the frequency multiplier to the reference input terminal and a phase of an output signal from the mixer, and outputting a phase difference signal;
a voltage control oscillator for generating a reference signal phase-synchronized with the signal under test based on the phase difference signal output from the phase comparator, and feeding the reference signal back to the reference signal input terminal of the frequency synthesized signal generator and the reference signal input terminal of the phase comparator; and
a sampling signal generator circuit for generating the sampling signal by employing the reference signal output from the voltage control oscillator.
In addition, in order to achieve the foregoing object, according to a thirteenth aspect of the present invention, there is provided a waveform measuring apparatus according to the twelfth aspect, further comprising:
a power divider for power dividing the signal under test that is an optical signal in two ways, if the signal under test is the optical signal; and
a clock recovery for detecting a clock with its repetition cycle from one of the signal under test divided by the wave divider, thereby outputting the signal under test that is the optical signal to be converted into a signal under test of an electrical signal of a sine waveform having the repetition frequency.
In addition, in order to achieve the foregoing object, according to a thirteenth aspect of the present invention, there is provided a waveform measuring apparatus according to the twelfth aspect, further comprising:
a power divider for, when the signal under test is an optical signal, power dividing the signal under test that is the optical signal into two ways; and
a clock recovery for detecting a clock of the repetition cycle from such one signal under test divided by the power divider, thereby converting the signal under test that is the optical signal into a signal under test of an electrical signal with a sine waveform having the repetition frequency, and outputting the converted signal.
In addition, in order to achieve the foregoing object, according to a fourteenth aspect of the present invention, there is provided a waveform measuring apparatus according to the thirteenth aspect, further comprising:
a photo detector for, when the sampling section is an electro-absorption modulator, light receiving a signal under test obtained by sampling a signal under test of the optical signal input from the power divider by means of a sampling signal input from the sampling signal generator circuit, and converting a signal under test that is an optical signal after sampled into a signal under test of an electrical signal;
an analog/digital converter for converting the signal under test converted into the electrical signal by means of the photo detector into a digital signal under test, and delivering the converted digital signal under test to the data processing section; and
a display for converting a scale of a time axis of the enveloped waveform obtained by the data processing section into a scale of an original signal under test, and displaying a signal waveform of the signal under test.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.